The present invention relates to a spindle motor for use in magnetic disk devices, optical disk devices and the like, and more particularly to a method of assembling a plain bearing unit suitable for accuracy and high speed rotation and coping with mass production.
Recently, a disk driving spindle motor for use in magnetic disk devices and optical disk devices is required to have the performance of high speed and accurate rotation. A motor employing a conventional ball bearing has a limit in the performance of high speed and accurate rotation when improving the machining accuracy, lubrication and sound noise of the ball bearing. And, a motor on which a hydrodynamic plain bearing is mounted has been proposed and brought into practical use. As a hydrodynamic plain bearing, JP-A-10-73126 and JP-A-8-80091 disclose a hydrodynamic groove bearing that has a bearing surface formed with shallow grooves for generating a dynamic pressure. Furthermore, JP-A-9-303398 and JP-A-11-96658 disclose a bearing unit using a three lobe bearing with a bearing surface comprising circular arc surfaces that have a dynamic pressure generating function.
It is necessary for a motor using the above described hydrodynamic bearing to suppress the axial shake to one xcexcm to sub xcexcm or less in order to keep the accurate rotation. For this, a shaft with a high bearing rigidity is required, and it is necessary to adopt a hydrodynamic bearing.
This hydrodynamic bearing, in the case of the hydrodynamic groove bearing, has a plurality of shallow grooves of several xcexcm are provided in its bearing surface in order to maintain the bearing rigidity and generates a dynamic pressure on the bearing surface through the pumping action due to the rotation of a shaft to attain its object. In the case of the three lobe bearing, a space defined between a shaft and the bearing surface is formed in a wedge-like shape, and the bearing generates a dynamic pressure through the wedge action of an oil film, while maintaining the bearing rigidity. In either bearing, in order to raise the bearing rigidity, the space between the shaft and the bearing surface has a size on the order of xcexcm. Furthermore, it is necessary for keeping the bearing rigidity thus designed to surely enclose lubricating oil in the bearing part.
However, it is impossible to easily enclose the lubricating oil in such a narrow space without containing any air bubbles. In view of this, JP-A-10-73126 takes a measure of once putting an assembled motor in a vacuum chamber and enclosing lubricating oil by utilizing a capillary attraction or the external atmospheric pressure while evacuating the interior of the machine. By this method, it becomes possible to fill the interior of the bearing with lubricating oil in the state of a low air content, but no concrete enclosing method is described. More specifically, this type of motor is answerable to the mass production and is also related to the production cost. Therefore, the enclosing method of lubricating oil is a key technique in the manufacturing of the motor. When the lubricating oil is poured in the state where the interior of the machine is merely evacuated, there is a possibility that air bubbles dissolved in the lubricating oil will expand into a popping state. The above described reference fails to describe how to pour oil while preventing its popping.
The embodiment disclosed in JP-A-8-80091 takes a measure of providing in the shaft a lubricating oil supply hole for supplying the lubricating oil into the interior of the bearing, but it fails to describe any measure for surely removing air bubbles. Similarly to the above described embodiment, there is specifically described no concrete method for filling the interior of the bearing with lubricating oil in the state of a low air content.
As concerns the embodiment disclosed in JP-A-9-303398, description has been made on the amount of oil enclosed, but as for the inclusion of air bubbles, no description is given, and as for the removal of air bubbles in the bearing unit or the like, there is also no consideration.
In the embodiment disclosed in JP-A-11-96658, there is described a structure for using magnetic fluid as lubricating oil and magnetizing the bearing and the shaft to hold the lubricating oil on the bearing surface. But, no description is made concerning any method of supplying the magnetic fluid and the inclusion of air bubbles, and there is also no consideration as for the removal of air bubbles in the bearing unit.
The present invention has been accomplished in view of the defects of the prior art above described. It is an object of the invention to provide an oiling method for a bearing unit, which surely feeds lubricating oil with little air content to a bearing unit of a disk driving motor or the like, and which can cope with mass production motors.
According to the invention, as means for attaining the above object, magnetic fluid is used as lubricating oil in a motor using a hydrodynamic bearing. And, as the first concrete enclosing method, the magnetic fluid of a specified amount is introduced and held in the bearing unit by arranging a permanent magnet outside the bearing unit before inserting a shaft therein and by using the magnetic attraction force. The shaft is then inserted, and after that, the magnetic fluid is supplied into the narrow space of the above described bearing part by a capillary attraction. By this lubricating oil enclosing method, not only the magnetic fluid can be held in the bearing part by the magnetic attraction force and the surface tension in the bearing space, but also air bubbles can be removed from the magnetic fluid by the difference in density between the magnetic fluid and the air. Accordingly, after that, the permanent magnet for attraction of the magnetic fluid is removed to release the magnetic attraction force acting on the magnetic fluid, and the motor is assembled.
The second concrete enclosing method is as follows. It is different from the first method in that a permanent magnet for holding the magnetic fluid is arranged between radial bearings. Besides, the permanent magnet provided outside the bearing unit has a magnetic force larger than that of the permanent magnet for holding the magnetic fluid. By this method, it is possible to surely hold the magnetic fluid in the bearing part by the magnetic attraction force of the magnetic fluid holding permanent magnet after the enclosing of the magnetic fluid.
The third concrete enclosing method is the same as the first method and the second method in that magnetic fluid is used as lubricating oil. In this method, the magnetic fluid of a predetermined amount is previously held by an magnetic attraction force on a thrust bearing, which serves as a member for closing one end of the bearing unit, or on a cover (non-magnetic material), and the thrust bearing or the cover is attached after incorporating a radial bearing and a rotary shaft into the bearing unit. After that, the magnet provided on the thrust bearing side is removed to release the magnetic attraction force, and the magnetic fluid is supplied in the narrow space of the bearing part.